With the growth of the wireless communications industry, wireless communications protocols have become more sophisticated. Communications systems may have to provide support for multiple communications protocols. One such system is the Universal Mobile Telecommunications System (UMTS), which may require support for both Wide Band Code Division Multiple Access (WBCDMA) and Enhanced General Packet Radio Service (EGPRS) communications protocols. These two protocols have many differences such that two different RF receiver architectures may be needed. RF receivers are often battery powered and must function with minimal power consumption, cost, and space. As a result, there is a need for a single radio receiver that can efficiently operate in at least two different operating modes, using two different receiver architectures.
A traditional RF receiver architecture is the super-heterodyne architecture in which a received RF signal is mixed with a local oscillator signal to obtain a lower intermediate frequency (IF) signal. The IF signal is then filtered to the desired channel bandwidth to remove interfering signals and signals from adjacent channels. As channel bandwidths become narrower, the inclination is to reduce the frequency of the IF signal. As a result, receivers using a very low intermediate frequency (VLIF) for their IF sections are becoming increasingly common for certain communications protocols; however, some image frequencies may not be removed with upstream RF bandpass filtering. Another example is a direct-conversion receiver, which has a direct current (DC) IF signal; however, problems with 1/f noise, DC offsets, and second-order inter-modulation (IIP2) effects may eliminate the direct-conversion receiver from some applications. The WBCDMA protocol lends itself to direct-conversion, but the EGPRS protocol lends itself to VLIF. A receiver with a different receive path for each protocol could be used in a UMTS system; however, since the WBCDMA protocol and the EGPRS protocol do not operate simultaneously, a receiver with a single receive path for both protocols could reduce cost, complexity, and current consumption.
One design challenge in a VLIF receiver is rejection of image frequencies. In any heterodyne receiver, when a received RF input signal FR, mixes with a local oscillator signal FLO, the mixer produces an output signal with sums and differences of FR and FLO. Specifically, the frequencies of FR−FLO, FLO−FR, and FR+FLO are the dominant mixer output frequency combinations. If FLO is chosen with a lower frequency than a desired RF input signal FDRF, then the FR−FLO portion of the mixer output signal produces a wanted VLIF signal FDVLIF; however, the mixer output signal will also include an FR+FLO image signal, which is close to double the frequency of FDRF and easily removed by IF bandpass filtering. If a blocking image signal FBIS with a frequency located at a frequency of FLO minus the frequency of FDVLIF is received, the FLO−FR portion of the mixer output will produce an image that is identical in frequency with FDVLIF, and cannot be removed with normal IF filtering techniques; therefore, if upstream RF bandpass filtering cannot remove the blocking image signal, then other techniques must be used to remove the signal. However, since the blocking signal is phase-shifted by 180 degrees from FDVLIF, a quadrature receiver architecture can be used to filter out the blocking image signal. A quadrature receiver architecture uses two mixers receiving the same RF input signal, which is mixed with two different local oscillator signals that are equal in frequency and phase-shifted from each other by 90 degrees. Complex filtering methods can then be used to filter out the blocking image signal. Any mismatch between the processing of in-phase signals and quadrature-phase signals will result in degradation of the rejection of image signals.
There is a special situation in which a frequency of the wanted VLIF signal FDVLIF, called the wanted VLIF frequency, is less than the frequencies of blocking image signals. In this special situation, there is a benefit to reducing the wanted VLIF frequency, namely improved image rejection; however, a lower frequency increases 1/f noise, DC offsets, and IIP2 problems, which reduces the effective sensitivity of the receiver. In some networks, there is a loose correlation between the signal strength of a desired signal and the signal strength of interfering image signals; therefore, when the signal strength of a desired signal is small, a higher VLIF frequency is desirable to increase receiver sensitivity. The resulting reduced image rejection is acceptable, since the signal strengths of interfering image signals are also small. Likewise, when signal strengths of interfering image signals are large, a larger VLIF frequency is desirable to increase image rejection. The resulting reduced receiver sensitivity is acceptable, since the signal strength of the desired signal is also large; therefore, in some networks, it would be beneficial to have an inverse correlation of the VLIF frequency with signal strength.
Given the above factors, a need exists for a quadrature single-path receiver that can support both direct conversion and VLIF modes of operation, can adjust the VLIF frequency based on received signal strength, and can effectively reject image interfering signals with filtering, matching between the circuitry processing the in-phase signals and the quadrature-phase signals, or both.